Encapsulated ceramic armor

ABSTRACT

An impact resistant clad composite armor which includes a ceramic core, and a layer of bulk amorphous alloy surrounding the ceramic core and preferably bonded chemically to the ceramic core and a method of manufacturing such armor is provided.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority on U.S. Application No. 60/363,389,filed Mar. 11, 2002, the disclosure of which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to the encapsulation of ceramic materialswith bulk amorphous alloys and, more particularly, to an impactresistant ceramic armor encapsulated with bulk amorphous alloys and amethod of forming such armor.

BACKGROUND OF THE INVENTION

Ceramic materials have long been considered for use in the fabricationof armor components because ceramic materials have a high hardness, arepotentially capable of withstanding armor-piercing projectiles, and arerelatively lightweight. However, the use of ceramic materials in armorapplications has been limited by the low impact resistance of thesematerials, which results from ceramic's brittleness and lack oftoughness. Indeed, one of the significant drawbacks to the use ofceramic materials in armor applications is that they lack repeat hitcapability. In other words, ceramic materials tend to disintegrate uponthe first hit and cease to be useful when subjected to multipleprojectiles. For a more effective utilization of ceramic materials inarmor applications, it is necessary to improve the impact resistance ofthis class of materials.

One method to overcome the disintegration of ceramic armors is toencapsulate the ceramic with a layer of surrounding metal. Such a methodis disclosed in U.S. Pat. No. 4,987,033 incorporated herein byreference. However, there are still deficiencies with the encapsulationof ceramic cores in the prior art. First, because of the properties ofthe proposed metals, conventional casting processes cannot be readilyand effectively utilized to encapsulate the ceramic cores. For example,the very high solidification shrinkage of metals (˜6 to 12%) precludesthis process as the encapsulating metal exerts undue stresses on theceramic core and can result in the fracturing of the ceramic. As suchmore expensive encapsulation processes, such as, powder metallurgytechniques are used as disclosed in U.S. Pat. No. 4,987,033.

Accordingly, a need exists for an armor component formed of anencapsulated ceramic material that has improved impact resistance, andfor an inexpensive method for forming an armor component from a ceramicmaterial that has improved impact resistance.

SUMMARY OF THE INVENTION

The invention, as embodied and broadly described herein, is directed toan impact resistant composite armor which includes a ceramic core, and alayer of bulk amorphous alloy surrounding the ceramic core andpreferably bonded chemically to the ceramic core.

In another embodiment, the invention is also directed to a method offorming an encapsulated ceramic armor from an amorphous alloy material.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bebetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic of a first exemplary embodiment of a compositearmor in accordance with the present invention.

FIG. 2 is a schematic of a second exemplary embodiment of a compositearmor in accordance with the present invention.

FIG. 3 is a schematic of a third exemplary embodiment of a compositearmor in accordance with the present invention.

FIG. 4 is a schematic of a fourth exemplary embodiment of a compositearmor in accordance with the present invention.

FIG. 5 is a schematic of a fifth exemplary embodiment of a compositearmor in accordance with the present invention.

FIG. 6 is a graphical comparison of the yield strength properties ofconventional metals and bulk amorphous alloys according to the presentinvention.

FIG. 7 is a graphical comparison of the elastic limit properties ofconventional metals and bulk amorphous alloys according to the presentinvention.

FIG. 8 is a graphical depiction of the processing routes for bulkamorphous alloys according to the present invention.

FIG. 9 is a graphical comparison of the yield strength versus meltingtemperature of conventional metals and bulk amorphous alloys accordingto the present invention.

FIG. 10 is a graphical comparison of the solidification shrinkage ofconventional metals and bulk amorphous alloys according to the presentinvention.

FIG. 11 is a graphical comparison of the coefficient of thermalexpansion versus solidification temperature of conventional metals andbulk amorphous alloys according to the present invention.

FIG. 12 is a graphical depiction of the solidification properties ofbulk amorphous alloys according to the present invention.

FIG. 13 is a graphical depiction of the solidification properties ofbulk amorphous alloys according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention, is directed to a bulk amorphous alloy encapsulated impactresistant composite armor. The armor includes a ceramic core, and alayer of bulk amorphous alloy surrounding the ceramic core andpreferably bonded chemically to the ceramic core.

Exemplary embodiments of composite armors in accordance with the presentinvention are chown schematically in FIGS. 1 to 5.

The geometry of the ceramic-amorphous alloy configuration can take avariety of forms. In one particular form, a monolithic single piece ofceramic material (a tile) is fully encapsulated by an amorphous alloymaterial, as shown in FIG. 1. In another form, one face of the ceramicarmor can be left exposed as shown in FIG. 2. The ceramic tile dimensioncan vary from 2″×2″ up to 30″×30″ or more depending on the availabilityand the quality of the specific ceramic material. The thickness of thetile can be from 0.1″ to 1.0″ or more. The tile geometry can be in anyshape such as rectangular, square, hexagonal, triangular. The use ofsuch single-piece monolithic ceramics is desired for highereffectiveness of armor.

In another embodiment of the invention, the amorphous alloy and theceramic material can be layered in a laminated structure, where thealternating layers of ceramic material and bulk amorphous are composedas shown in FIGS. 3 and 4.

In yet another embodiment, a plurality of ceramic pieces may be embeddedin an amorphous alloy matrix as shown in FIG. 5. In this case, thedimensions of the ceramic pieces are smaller than 2″×2″. Such aplurality of ceramic pieces facilitates the preservation of the generalstructure of the ceramic-amorphous alloy composite by limiting thepropagation of ceramic fracture throughout the whole structure as thecracks in ceramic will be arrested by the tougher metallic matrix. Suchconfigurations are also desirable for reduced cost, as well as to makinglarger structures when larger size tiles are not readily available for aparticular ceramic material. The shape of the ceramic pieces can be inany suitable form such as the cylindrical short rods shown in FIG. 5.Various configurations and shapes of ceramic pieces can be utilized asdisclosed in U.S. Pat. No. 6,408,734.

In any of the above embodiments of the invention, the thickness of theamorphous layer may be varied. In one such embodiment, the thickness ofthe amorphous layer is less than 2.0 mm and preferably less than 1.0 mm.In these cases, the toughness and ductility of the amorphous alloy willbe higher due to geometric constraints.

The ceramic core, according to the present invention, is formed in theshape of the desired armor component. The ceramic core preferably iscomprised of a ceramic material selected from the group consisting ofAl₂O₃, B₄C, SiC, Si₃N₄ and TiB₂. Preferably the ceramic material has adensity 99% or higher. Although specific ceramics are described above,it will be understood that the principles of the invention areapplicable to any ceramic material having high hardness.

The metallic outer coating is comprised of a bulk amorphous alloy. Suchbulk solidifying amorphous alloys are recently discovered family ofamorphous alloys, which may be cooled at substantially lower coolingrates, of about 500 K/sec or less, and substantially retain theiramorphous atomic structure. As such, these materials may be produced inthickness of 1.0 mm or more, substantially thicker than conventionalamorphous alloys, which require cooling rates of 10⁵ K/sec or more, andcan only be cast to thicknesses of around 0.020 mm. U.S. Pat. Nos.5,288,344; 5,368,659; 5,618,359; and 5,735,975 (whose disclosures areincorporated by reference in its entirety) disclose such bulksolidifying amorphous alloys. An exemplary family of bulk solidifyingamorphous alloys is described by the molecular formula:(Zr,Ti)_(a)(Ni,Cu, Fe)_(b)(Be,Al,Si,B)_(c), where a is in the range offrom about 30 to about 75, b is in the range of from about 5 to about60, and c in the range of from about 0 to about 50, in atomicpercentages. Furthermore, these alloys may accommodate substantialamounts of other transition metals up to 20%, in atomic percent, such asNb, Cr, V, Co. A preferable alloy family is(Zr,Ti)_(a)(Ni,Cu)_(b)(Be)_(c), where a is in the range of from about 40to about 75, b is in the range of from about 5 to about 50, and c in therange of from about 5 to about 50 in atomic percentages. Still, a morepreferable composition is (Zr,Ti)_(a)(Ni,Cu)_(b)(Be)_(c), where a is inthe range of from about 45 to about 65, b is in the range of from about7.5 to about 35, and c in the range of from about 10 to about 37.5 inatomic percentages. Another preferable alloy family is(Zr)_(a)(Nb,Ti)_(b) (Ni,Cu)_(c)(Al)_(d), where a is in the range of fromabout 45 to about 65, b is in the range of from about 0 to about 10, cis in the range of from about 20 to about 40 and d in the range of fromabout 7.5 to about 15, in atomic percentages.

Such bulk solidifying amorphous alloys may sustain strains up to 1.5% ormore without any permanent deformation or breakage. Further, thesematerials have high fracture toughness of 10 ksi-√in or more, andpreferably 20 ksi-√in or more. Also, they have high hardness values of 4GPa or more, and preferably 5.5 GPa or more. The yield strength level ofbulk solidifying alloys range from 1.6 GPa and reach up to 2 GPa andmore exceeding the current state of the Titanium alloys. Furthermore,the above bulk amorphous alloys have a density in the range of about 4.5to 6.5 g/cc, and, as such, provide high strength to weight ratios. Inaddition, bulk solidifying amorphous alloys have very good corrosionresistance.

Another set of bulk-solidifying amorphous alloys are ferrous-based metal(Fe, Ni, Co) compositions. Examples of such compositions are disclosedin U.S. Pat. No. 6,325,868, and in publications to A. Inoue et. al.,Appl. Phys. Lett., Volume 71, p 464 (1997); Shen et. al., Mater. Trans.,JIM, Volume 42, p 2136 (2001); and Japanese patent application2000126277 (Publ. #0.2001303218 A), all of which are incorporated hereinby reference. One exemplary composition of such ferrous-based alloys isFe₇₂Al₅Ga₂P₁₁C₆B₄. Another exemplary composition of such alloys isFe₇₂Al₇Zr₁₀Mo₅W₂B₁₅. Although, these alloy compositions are notprocessable to the degree of the Zr-base alloy systems, they can bestill be processed in thicknesses around 1.0 mm or more, sufficient tobe utilized in the current invention. In addition, although theferrous-based alloys have generally higher densities, from 6.5 g.cc to8.5 g/cc, their yield strength is also higher, ranging from 2.5 GPa to 4GPa or more, making them particularly attractive for armor applications.Similarly, ferrous-based alloys have elastic strain limits of higherthan 1.2% and generally about 2.0%. Ferrous metal-base bulk amorphousalloys also have very high yield hardnesses ranging from 7.5 GPA to 12GPa.

In general, crystalline precipitates in bulk amorphous alloys are highlydetrimental to these materials' properties, especially to the toughnessand strength. As such, it is generally preferred to keep the volumefraction of crystalline precipitates to a minimum if possible. However,there are cases in which ductile crystalline phases precipitate in-situduring the processing of bulk amorphous alloys, and which are indeedbeneficial to the properties of bulk amorphous alloys, especially to thetoughness and ductility. Such bulk amorphous alloys comprising suchbeneficial precipitates are also included in the current invention. Oneexemplary case is disclosed in the publication to C.C. Hays et. al,Physical Review Letters, Vol. 84, p 2901, 2000, which is incorporatedherein by reference.

There are several advantages to using bulk amorphous alloys as asurrounding material in ceramic cores. First, the very high “yield”strength of bulk amorphous alloys can be utilized to constrain theceramic core very effectively and impede the material's disintegration.Herein, the important strength parameter is the “yield” strength of thesurrounding metal. When the armor package takes a hit, the ceramic corewill tend to fracture and dimensionally expand due to opening cracks. Inthis situation, the surrounding metal will be forced to stretch out andthe material's resistance to yielding will be an important factor inimpeding the disintegration of the ceramic core. As shown in FIG. 6,bulk amorphous alloys (indicated, throughout the figures as LM-001, anAmorphous alloy manufactured by LiquidMetal Technologies, Inc.) have ayield strength of 1.6 GPa and more, much higher than conventionalmetals. The higher the yield strength is the higher resistance againstdisintegration forces and provides a more effective constrain.

Further, elastic strain limit also plays a similarly important role, asa material having a higher elastic limit provides a more effectiveconstraint for the ceramic core. Accordingly, the higher the elasticlimit, the better the hold the material will have on the ceramic core.As shown in FIG. 7, generally conventional metals have elastic strainlimits of 0.6% or less. In contrast, bulk amorphous alloys have elasticstrain limits higher than 1.2% and generally their elastic strain limitis about 2.0%. The unique combination of high yield-strength and highelastic limit of bulk amorphous metals makes them highly useful as asurrounding metal for ceramic cores since highly effective constraintsfor the ceramic core can be achieved and the integrity of the ceramicmaterial can be preserved for multiple hits.

Another advantage of the bulk solidifying amorphous alloys is thedeformation mechanism by localized shear bands. In the case of ceramicencapsulation, the deformation of amorphous alloys will be limited to avery small portion of the amorphous alloy, which will in effect keep thesubstantial portion of the amorphous alloy intact. This will allow theamorphous alloy to preserve the constraining effect on the ceramic andimproving its effectiveness. In the case of conventional metal andalloys, the deformation of metallic component propagates throughout themost the structure and as such distorts and deprives the constrainingaction necessary to improve the effectiveness of ceramic component.

The current invention also provides a method for forming the bulkamorphous alloy encapsulated ceramic core. Further advantages of usingbulk amorphous alloys as the surrounding material can be seen from themethod of forming the bulk amorphous alloy encapsulated ceramic core.The methods described in the invention also provide a cost-effectiveprocess for encapsulation of ceramic cores with a very highyield-strength metal. Further, the methods provide a near-to-net shapeforming process for the encapsulation of ceramic cores.

In one embodiment of the invention, the bulk amorphous alloy in moltenform (above the melting temperatures of its crystalline phases) may bedisposed so as to surround the ceramic core in a suitable mold by acasting method (route A as shown in FIG. 8). Various casting methods canbe used such as permanent mold casting, counter gravity casting anddie-casting. One such process, die-casting, is disclosed in U.S. Pat.No. 5,711,363, which is incorporated herein by reference. The bulkamorphous alloy is preferably disposed to surround the ceramic coreuniformly so that a layer having uniform thickness will be formed uponsolidification of the molten alloy. The amount of bulk amorphous alloydisposed around the ceramic core may be varied depending on the desiredthickness of the layer.

It should be noted that the above-mentioned casting operations are notgenerally available for high yield-strength materials and only the useof bulk-solidifying amorphous alloys allows the use of such castingprocesses in manufacturing encapsulated ceramics with very high-yieldstrength metals. As shown in FIG. 9 with the example of LM-001 (aZr-base alloy) and other ordinary alloys, bulk-solidifying amorphousalloys exhibit a very high yield-strength compared to their meltingtemperatures. The high temperature melt processing degrades theproperties of ceramics and the formation reaction products at theinterface not only weakens the overall composite structure but alsofacilitates the formation and propagation of cracks in ceramic material.Further, as shown in FIG. 10, the solidification shrinkage ofconventional metals is very high, which causes the fracture of ceramiccomponents due to differential thermal stresses between the conventionalmetals and ceramics.

As detailed in FIGS. 11 to 13, bulk amorphous alloys retain theirfluidity from above the melting temperature down to the glass transitiontemperature due to the lack of a first order phase transition. This isin direct contrast conventional metals and alloys. Since, bulk amorphousalloys retain their fluidity, they do not accumulate significant stressfrom their casting temperatures down to below the glass transitiontemperature. As such, these characteristics of bulk amorphous alloys canbe utilized to exert a highly beneficial compressive stress on theceramic core. Such beneficial compressive stress makes the ceramic coreto defeat the projectiles much more effectively by delaying theformation of cracks in the ceramic material.

The advantage of such compressive stresses is two-fold. First, theceramic component will have a higher tensile strength and will be moreeffective in defeating the projectile as the projectile will spend moretime (and energy) before it causes the ceramic component to developcracks and fail. (Tensile stresses are the cause of the prematurefailure in ceramic components since in general ceramics have higherstrength in compression than in tension.). This can allow the compositestructure to defeat projectiles without any damage to the ceramiccomponent and, therefore will allow the structure to take multiple hits.Secondly, after the defeat of first projectile, the compressive stresseson the ceramic component will be preserved due to the high elasticstrain limit of amorphous alloy. As such, the ceramic will be held inplace securely and evenly in case some damage, in the form of cracks,develop in the ceramic component. Even though the effectiveness of thesystem will be reduced (in the case of formation of cracks in ceramic),the compressive stresses will maintain the effectiveness of the ceramicfor subsequent hits, and at the minimum will keep the un-cracked portionof ceramic in place to defeat the projectile and dissipate its energy.

The compressive stresses are achieved by the differential thermalstresses generated through the different thermal expansion coefficientsof the amorphous alloy and the ceramic material. Since ceramic materialshave much lower coefficient of thermal expansion than the metallicalloys, the surrounding amorphous alloy will shrink more than theceramic and apply a hydrostatic compressive stresses. Accordingly, inone embodiment of the invention, the ceramic core is left under acompressive stress of 400 MPa or more and more preferably under acompressive stress of 800 MPa or more.

The bonding at the interface of bulk-solidifying amorphous alloy andceramic can take several forms. In one embodiment, the interface hasintimate contact mechanically, but not necessarily chemically (ormetalurgically). Such interfaces can be achieved with the belowmentioned molding process. In another embodiment, a chemically bondedinterface can be achieved. Such above described melt-processing can beeffectively utilized to form such interfaces. The very low meltingtemperatures of bulk-solidifying amorphous alloys makes it possible toutilize a time-temperature window to control the melt reactivity to formchemical bonds with minimal reaction products. For example, Zr—Ti-basebulk-solidifying amorphous alloys have typical melting temperatures onthe order of 650° C. to 850°, much lower than ordinary high-strength Tibase alloys (˜1,600° C.). In such operations, it is desirable to heatthe ceramic component to temperatures about the melt processingtemperatures. In a casting process, to encapsulate the ceramic, time andtemperature of casting parameters can be selected such that a chemicalbond between the surrounding bulk amorphous alloy and ceramic core canbe established. Zr—Ti base amorphous alloys are especially preferred forencapsulating ceramic components with chemically bonded interfaces.Herein, Zr—Ti base is defined as amorphous alloy composition having atotal of Zr and Ti greater than 30% atomic percentage.

Although casting processes are discussed above, in another embodiment ofthe invention, a feedstock of bulk amorphous alloy as a solid piece inthe amorphous form is provided and heated to about glass transitiontemperature or above (route B in FIG. 8). Subsequently, the bulkamorphous alloy, about the glass transition temperature or above, may bedisposed so as to surround the ceramic core in a suitable molding andthermo-plastic process. A variety of molding operations can be utilizedsuch as blow molding (clamping a portion of feedstock material andapplying a pressure difference on opposite faces of the unclamped area),die-forming (forcing feedstock material into a die cavity), andreplication of surface features from a replicating die. U.S. Pat. Nos.6,027,586; 5,950,704; 5,896,642; 5,324,368; and 5,306,463 (each of whosedisclosures is incorporated by reference in its entirety) disclosemethods to form molded articles of amorphous alloys exploiting the glasstransition properties of the materials. In such an embodiment, the bulkamorphous alloy is preferably disposed to surround the ceramic coreuniformly so that a layer having uniform thickness will be formed uponsolidification of the molten alloy. The amount of bulk amorphous metaldisposed around the ceramic core may be varied depending on the desiredthickness of the layer. Such processes allow to form a very intimatemechanical contact between the ceramic and amorphous alloy, and as suchprovides a more effective constraint to the ceramic than other methodsthat do not involve such interface diffusion bonding. It should beunderstood that the feedstock of amorphous alloy can also be in powderform and during the molding operation the powder can be consolidatedaround the ceramic core.

Although subsequent processing steps may be used to finish the amorphousalloy articles of the current invention, it should be understood thatthe mechanical properties of the bulk amorphous alloys and compositescan be obtained in the as cast and/or molded form without any need forsubsequent process such as heat treatment or mechanical working. Inaddition, in one embodiment the bulk amorphous alloys and theircomposites are formed into complex near-net shapes in the two-stepprocess. In such an embodiment, the precision and near-net shape ofcasting and moldings is preserved.

Although specific embodiments are disclosed herein, it is expected thatpersons skilled in the art can and will design alternative amorphousalloy encapsulated ceramic armor devices and methods to produce theamorphous alloy encapsulated ceramic armor devices that are within thescope of the following claims either literally or under the Doctrine ofEquivalents.

1. A ceramic armor comprising: a ceramic core; and a metallic layerformed from an amorphous alloy having a yield strength of at least 1.6GPa and an elastic strain limit of at least 1.2%, the amorphous alloyencapsulating the ceramic core such that the metallic layer places theceramic core under a compressive stress of at least 400 MPa, wherein atleast a portion of the metallic layer formed from an amorphous alloy hasa thickness of about 0.5 mm or more.
 2. The ceramic armor as describedin claim 1, wherein the amorphous alloy is described by the followingmolecular formula: (Zr,Ti)_(a)(Ni,Cu, Fe)_(b)(Be,Al,Si,B)_(c), wherein“a” is in the range of from about 30 to 75, “b” is in the range of fromabout 5 to 60, and “c” in the range of from about 0 to 50 in atomicpercentages.
 3. The ceramic armor as described in claim 1, wherein theamorphous alloy is described by the following molecular formula:(Zr,Ti)_(a)(Ni,Cu)_(b)(Be)_(c), wherein “a” is in the range of fromabout 40 to 75, “b” is in the range of from about 5 to 50, and “c” inthe range of from about 5 to 50 in atomic percentages.
 4. The ceramicarmor as described in claim 1, wherein the amorphous alloy is a Zr—Tibase bulk solidifying amorphous alloy.
 5. The ceramic armor as describedin claim 1, wherein the amorphous alloy is described by the followingmolecular formula: (Zr)_(a)(Nb,Ti)_(b)(Ni,Cu)_(c)(Al)_(d), wherein “a”is in the range of from about 45 to 65, “b” is in the range of fromabout 0 to 10, “c” in the range of from about 20 to 40, and “d” in therange of from about 7.5 to 15 in atomic percentages.
 6. The ceramicarmor as described in claim 1, wherein the amorphous alloy can sustainstrains up to 1.5% or more without any permanent deformation orbreakage.
 7. The ceramic armor as described in claim 1, wherein theamorphous alloy has a high fracture toughness of at least about 10ksi-√in.
 8. The ceramic armor as described in claim 1, wherein theamorphous alloy has a high fracture toughness of at least about 20ksi-√in.
 9. The ceramic armor as described in claim 1, wherein theamorphous alloy has a high hardness value of at least about 4 Gpa. 10.The ceramic armor as described in claim 1, wherein the amorphous alloyhas a high hardness value of at least about 5.5 GPa.
 11. The ceramicarmor as described in claim 1, wherein the amorphous alloy has a densityin the range of about 4.5 to 6.5 g/cc.
 12. The ceramic armor asdescribed in claim 1, wherein the ceramic core is a material selectedfrom the group consisting of Al₂O₃, B₄C, SiC, Si₃N₄ and TiB₂.
 13. Theceramic armor as described in claim 1, wherein the ceramic core has adensity of 99% or higher.
 14. The ceramic armor as described in claim 1,wherein the amorphous alloy is based on ferrous metals wherein thehardness of the amorphous alloys is about 7.5 Gpa and higher.
 15. Theceramic armor as described in claim 1, wherein the amorphous alloyfurther comprises a ductile metallic crystalline phase precipitate. 16.The ceramic armor as described in claim 1, wherein the metallic layerfully surrounds the ceramic core.
 17. The ceramic armor as described inclaim 1, wherein the metallic layer has a substantially uniformthickness.
 18. The ceramic armor as described in claim 1, wherein themetallic layer is chemically bonded to the ceramic core.
 19. The ceramicarmor as described in claim 1, wherein the metallic layer applies acompressive stress of 800 Mpa or more to the ceramic core.
 20. Theceramic armor as described in claim 1, wherein at least one face of theceramic core is exposed.
 21. The ceramic armor as described in claim 1,further comprising a plurality of independent ceramic cores at leastpartially encapsulated by the metallic layer.
 22. The ceramic armor asdescribed in claim 1, comprising a plurality of alternating layers ofthe ceramic core and the metallic layer.
 23. The ceramic armor asdescribed in claim 1, wherein the metallic layer is less than 2.0 mm.